This patent application is directed at a current collector that works with an anode or a cathode of a lithium cell (e.g. lithium-ion cell, lithium-metal cell, or lithium-ion capacitor), a supercapacitor, a non-lithium battery (such as the zinc-air cell, nickel metal hydride battery, sodium-ion cell, and magnesium-ion cell), and other electrochemical energy storage cells. This application is not directed at the anode or the cathode itself.
The lithium-metal cell includes the conventional lithium-metal rechargeable cell (e.g. using a lithium foil as the anode and MnO2 particles as the cathode active material), lithium-air cell (Li-Air), lithium-sulfur cell (Li—S), and the emerging lithium-graphene cell (Li-graphene, using graphene sheets as a cathode active material), lithium-carbon nanotube cell (Li—CNT, using CNTs as a cathode), and lithium-nano carbon cell (Li—C, using nano carbon fibers or other nano carbon materials as a cathode). The anode and/or the cathode can contain some lithium, or can be prelithiated prior to or immediately after cell assembly.
Rechargeable lithium-ion (Li-ion), lithium metal, lithium-sulfur, and Li metal-air batteries are considered promising power sources for electric vehicle (EV), hybrid electric vehicle (REV), and portable electronic devices, such as lap-top computers and mobile phones. Lithium as a metal element has the highest lithium storage capacity (3,861 mAh/g) compared to any other metal or metal-intercalated compound as an anode active material (except Li4.4Si, which has a specific capacity of 4,200 mAh/g). Hence, in general, Li metal batteries (having a lithium metal anode) have a significantly higher energy density than conventional lithium-ion batteries (having a graphite anode).
Historically, rechargeable lithium metal batteries were produced using non-lithiated compounds having relatively high specific capacities, such as TiS2, MoS2, MnO2, CoO2, and V2O5, as the cathode active materials, which were coupled with a lithium metal anode. When the battery was discharged, lithium ions were transferred from the lithium metal anode to the cathode through the electrolyte and the cathode became lithiated. Unfortunately, upon repeated charges and discharges, the lithium metal resulted in the formation of dendrites at the anode that ultimately caused internal shorting, thermal runaway, and explosion. As a result of a series of accidents associated with this problem, the production of these types of secondary batteries was stopped in the early 1990's giving ways to lithium-ion batteries. Even now, cycling stability and safety concerns remain the primary factors preventing the further commercialization of Li metal batteries (e.g. Lithium-sulfur and Lithium-transition metal oxide cells) for EV, HEY, and microelectronic device applications.
Prompted by the aforementioned concerns over the safety of earlier lithium metal secondary batteries led to the development of lithium-ion secondary batteries, in which pure lithium metal sheet or film was replaced by carbonaceous materials (e.g. natural graphite particles) as the anode active material. The carbonaceous material absorbs lithium (through intercalation of lithium ions or atoms between graphene planes, for instance) and desorbs lithium ions during the re-charge and discharge phases, respectively, of the lithium-ion battery operation. The carbonaceous material may comprise primarily graphite that can be intercalated with lithium and the resulting graphite intercalation compound may be expressed as LixC6, where x is typically less than 1 (with graphite specific capacity <372 mAh/g).
Although lithium-ion (Li-ion) batteries are promising energy storage devices for electric drive vehicles, state-of-the-art Li-ion batteries have yet to meet the cost, safety, and performance targets (such as high specific energy, high energy density, good cycle stability, and long cycle life). Li-ion cells typically use a lithium transition-metal oxide or phosphate as a positive electrode (cathode) that de/re-intercalates Li+ at a high potential with respect to the carbon negative electrode (anode). The specific capacity of lithium transition-metal oxide or phosphate based cathode active material is typically in the range of 140-170 mAh/g. As a result, the specific energy (gravimetric energy density) of commercially available Li-ion cells featuring a graphite anode and a lithium transition-metal oxide or phosphate based cathode is typically in the range of 120-220 Wh/kg, most typically 150-200 Wh/kg. The corresponding typical range of energy density (volumetric energy density) is from 300 to 400 Wh/L. These specific energy values are two to three times lower than what would be required in order for battery-powered electric vehicles to be widely accepted.
A typical battery cell is composed of an anode current collector, an anode electrode (typically including an anode active material, a conductive filler, and a binder resin component), an electrolyte/separator, a cathode electrode (typically including a cathode active material, a conductive filler, and a binder resin), a cathode current collector, metal tabs that are connected to external wiring, and casing that wraps around all other components except for the tabs. The sum of the weights and the sum of the volumes of these components are the total cell weight and total cell volume, respectively. The total amount of energy stored by a cell is governed by the amount of cathode active material and the corresponding amount of anode active material. The specific energy and energy density of a cell is then defined as the total amount of energy stored by the total cell weight and cell volume, respectively. This implies that one way to maximize the specific energy and energy density of a cell is to maximize the amounts of active materials and to minimize the amounts of all other components (non-active materials), under the constraints of other battery design considerations.
In other words, the current collectors at the anode and the cathode in a battery cell are non-active materials, which must be reduced in order to increase the gravimetric and volumetric energy densities of the battery. Current collectors, typically aluminum foil (at the cathode) and copper foil (at the anode), account for about 15-20% by weight and 10-15% by cost of a lithium-ion battery. Therefore, thinner, lighter foils would be preferred. However, there are several major issues associated with state-of-the-art current collectors:                (1) Due to easy creasing and tearing, thinner foils tend to be more expensive and harder to work with.        (2) Due to technical constraints, it is difficult, if not impossible, to fabricate metal foils thinner than 10 μm (e.g. Cu) or thinner than 20 μm (e.g. Al, Ni, stainless steel foil) in mass quantities.        (3) Current collectors must be electrochemically stable with respect to the cell components over the operating potential window of the electrode. In practice, continued corrosion of the current collectors can lead to a gradual increase in the internal resistance of the battery, resulting in persistent loss of the apparent capacity.        (4) Oxidation of metal current collectors is a strong exothermic reaction that can significantly contribute to thermal runaway of a lithium battery.Accordingly, the current collectors are crucially important for cost, weight, safety, and performance of a battery. Instead of metals, graphene has been considered as a potential current collector material, as summarized in the references listed below.        